An example of a known bolt hole with threads is generally shown at 1 in a vertical slice view in prior art FIG. 1A. The bolt hole 1 is formed in a part 2 and extends inwardly from a chamfer 12 in a surface 2A of part 2. A bolt 100 shown in vertical cross-section is illustrated in prior art FIG. 1B. The bolt is for threading into the bolt hole 1. Chamfers, however, although preferred, are not required.
When the term “bolt” is used herein it should be understood that a screw is also included in the definition of the word “bolt”.
The term “tap” is used in connection with roll forming threads.
The term “root” of a thread in a hole means valley between adjacent threads and is the major diameter between thread valleys on opposing sides of the hole.
The term “crest” of a thread in a hole means the peak of the thread and is the minor diameter between thread peaks on opposing sides of the hole.
The term “root” of a thread at a periphery of a tap means the valley between adjacent threads and is the minor diameter between thread roots on opposing sides of the tap.
The term “crest” of a thread at a periphery of a tap means the peak of the thread and is the major diameter between thread peaks on opposing sides of the tap.
The bolt hole 1 has a first start thread 3 located at the chamfer 12, a second start thread 4, and a plurality of following threads 5. An inner or minor diameter 6 of the threads 4 and 5 between crests of the threads is constant while the minor diameter of the first thread 3 is slightly greater due to the chamfer thereat. An outer or major diameter 7 of the threads 3, 4, and 5 is the same. The first start thread 3 followed by the second start thread 4 engage with a corresponding first start thread 101 followed by a second start thread 102 on a leading end of the bolt 100 shown in FIG. 1B. The bolt 100 also has further threads 103. An inner or minor diameter 104 of the threads 101, 102, and 103 is constant as is an outer or major diameter 105 of threads 101, 102, and 103. The minor diameter of the bolt threads or the bolt hole threads is also known as a root diameter.
So-called “fishmouth” curved concave flats 8A defined by an inwardly curved concave profile are formed at the thread crests or minor diameters 6. These curved concave flats 8A shown enlarged in FIG. 1AA for the two start threads 3 and 4 and next thread 5 enclosed by the circle FIG. 1AA in FIG. 1A are present in all of the threads 3, 4, and 5 of the hole 1. When one attempts to thread the bolt 100 into the bolt hole 1, and particularly if done automatically, frequently the first and possibly the second start threads 101 and 102 on the bolt 100 will not properly engage with the first and possibly the second start threads 3 and 4 in the bolt hole 1 in view of the presence of the fish mouth 8A shaped as a concave minor diameter on the first and second start threads 3 and 4.
Explaining in more detail, as shown in the prior art hole 1 of FIG. 1A, the first thread 3, the second thread 4, and the following normal threads 5 each have the fishmouth or curved concave flat 8A at the crest of the respective threads. This fishmouth at the crest is shown enlarged in FIG. 1AA which is illustrating the start threads at circle FIG. 1AA of FIG. 1A. The fishmouth 8A at the crest has first and second raised corner portions 8AA and 8AB. Between these raised corner portions is a central depression 8AC. This concave depression 8AC is formed during tapping when material movement at thread side-walls 22A and 22B is pushed towards the crest 8A and causes the corner portions 8AA and 8AB to be raised, resulting in the depression 8AC between the corner portions. This fishmouth on the first two start threads 3 and 4 of the threaded prior art bolt hole 1 frequently inhibits proper engagement of the start threads 101 and 102 on the prior art bolt 100, particularly during automated assembly.
Manufacture of the prior art bolt hole 1 is explained with reference to prior art FIGS. 2A and 2B. In FIG. 2A a start hole 10 shown in a vertical slice has a cylindrical straight wall 11 terminating at a floor 9, which may be either straight or concave. Start hole 10 is drilled into the part 2 from the part surface 2A. The chamfer 12, which is a partial conical section, is provided at an entrance to the start hole 10 in known fashion. As indicated previously, however, although desirable, a chamfer is not required.
As shown in prior art FIG. 2B in a side view, a tap is generally shown at 13 which is used to create the threads 3, 4, 5 of FIG. 1A in start hole 10. Such a prior art tap 13 is defined by a cylindrical shank 14 having at one end a square cross-section tap drive section 15 which merges into the shank 14 and is drivable when received in a drive bit of a drive unit. The shank 14 has a thread section 13 which creates the desired threads 3, 4, 5 shown in prior art FIGS. 1A and 2C. FIGS. 1A and 2C show the same identical prior art hole 1.
Prior art FIG. 2D shows a vertical cross-section at a middle of the prior art tap 13 shown in prior art FIG. 2B. The thread section 13 is formed on enlarged diameter portion 18 of shank 14. The shank 14 merges into the tap drive section 15. The thread section 13 has a plurality of normal threads 20 and first and second start threads 20A and 20B having respective outer or major diameters 21A and 21B which are slightly smaller than an outer or major diameter 17A of normal threads 20 in thread section 13.